In recent years a considerable amount of research has been done on ultrasonic motors that use mechanical vibrations in the ultrasonic range as the drive source. The mechanical vibrations are created by piezoelectric elements that are caused to alternately expand and contract by the application of an alternating electric field. The magnitude of these vibrations is usually very small, but by applying resonance principles large gain can be obtained. The direction of the vibrations are normally mechanically rectified so as to obtain the required motor output.
In principle such motors have a number of advantages over conventional electromagnetic motors. For example, they are capable of operating at low speed and with a very high output torque. Another important advantage of piezoelectric motors is that unlike electromagnetic motors they are not sensitive to any ambient electromagnetic fields, thus they may be used in applications where conventional motors cannot be used without difficulty.